1. Field of the Invention
The present invention relates to a laser diode driving circuit, more particularly to a laser diode driving circuit which controls a laser so as to generate a predetermined light output stably and quickly.
2. Description of the Prior Art
In optical information processors including optical disc devices, optical communications devices, and others, use is made of a laser diode as a light source. In using a laser diode as a light source in such a device, it is important to generate a light output from the laser diode fast and stably.
When the forward current reaches a predetermined threshold value, the laser diode starts to oscillate and outputs laser light. Thereafter, the light output increases in proportion to the forward current. However, the changes in the light output from the laser diode are relatively large compared to the changes in the forward current. For this reason, there have been proposed in the past driving circuits which can prevent variations in the light output of the laser diode and stably control the light output at a predetermined level. Such a circuit is called an automatic power control (APC) circuit. However, a prior driving circuit is uneconomical due to the relative complexity of the circuit configuration which requires, for example, three operational amplifiers. Moreover, its response to the variations in the light output is relatively slow, resulting in an unsatisfactory high speed control.
In addition, a laser diode has a relatively large forward voltage drop for a forward current so that it is accompanied by a considerable temperature rise, and like many of the semiconductor elements, it has a temperature dependence. Ordinarily, temperature rise is handled by the provision of a cooling means. However, when for some reason there flows an excess current that exceeds a rated value, temperature rise cannot be handled appropriately by means of a cooling means alone, leading to a concern of a breakdown of the laser diode. Further, in spite of the requirement that the laser diode be able to generate stably a prescribed light output even under changes in the temperature, there are few laser diodes that give sufficient consideration to the temperature compensation so as to control stably the light output of the laser diode for variations in the temperature, so that it becomes also necessary to have a sufficient measure for the temperature compensation.